The overall goal is to understand the genetic and metabolic defects that produce hypercholesterolemia and atherosclrosis in man. Recently, we have succeeded in defining a new pathway for the control of cholesterol metabolism in human cells. This pathway, which we have called the low density lipoprotein (LDL) pathway, consists of an ordered sequence of events by which cells bind LDL at a high affinity surface receptor site, internalize the lipoprotein by endocytosis, and release the lipoprotein cholesterol for use in cellular membrane synthesis. The importance of the LDL pathway was heightened by the demonstration that mutations at two different steps in this sequence produce three important clinical disorders involving hypercholesterolemia and/or atherosclerosis in man. We now apply for a Program Project grant that will allow us to further pursue the integrated, multidisciplinary approach that has worked so successfully in the past. We propose to study the coordinate regulation of cholesterol and lipoprotein metabolism in a series of model systems, each of which has been previously shown to illustrate an important aspect of the LDL pathway. These model systems include cultured fibroblasts and lymphoid cells from normal humans and from patients with disorders of lipoprotein and sterol metabolism; cultured human arterial smooth muscle cells; freshly isolated human lymphocytes; cultured mouse adrenal cells; and intact rats and guinea pigs that have been rendered lipoprotein-deficient. In conducting the proposed studies, we will utilize a wide variety of investigative disciplines, including 1) clinical genetics, 2) tissue culture and cell biology, 3) enzymology and molecular biology, 4) electron microscopy, and 5) lipoprotein and lipid physiology.